A writer in a hard disk drive (HDD) preamplifier drives an inductive head by passing a write current (Iw) from the writer to the head. Current Iw toggles between +Iw and −Iw values as consecutive one's are written on the disk. If there is impedance mismatch between the writer output and the head, ringing in the Iw waveforms occurs when current Iw is settling after switching state. Customarily, there are two methods of “damping” out the Iw ringing. However, there are pros and cons associated with each method.
Conventional Methods
(1) Series Damping (FIG. 1):
Two resistors Rs are added at the output of the “H-bridge” writer. In general, Rs>Rh, where Rh is the write head resistance. For a low CLK signal, transistors M1 and M4 are turned on, and thus current Iw flows in the direction shown in the diagram. Transistors M2 and M3 are turned off during this time. During the next Iw cycle (with a high CLK signal), current Iw flows in the reverse direction.
Pros: If a large VDD is available, Iw=Iwref. In this way, a predictable Iw current value flows through the head because there is only one path for Iw to flow when it goes from transistor M1 to M4.
Cons: Loss of voltage headroom due to the voltage potential developed across the two Rs, given by ΔV=2Rs*Iw. As a result, Iw<Iwref due to channel modulation effect on transistors M5 and M6 caused by reduced Vds. In an extreme case, transistors M5 and M6 enter into triode region of operation. Then, these transistors cease to be true output current mirror devices for MREF; thus, Iw<<Iwref.
(2) Parallel Damping (FIG. 2):
Iw ringing can also be tamed by a resistor Rp placed in parallel with the head. Note that, in general, Rp>>Rh.
Pros: No “additional” voltage drop across the damping resistor Rp.
Cons: Part of current Iw, denoted by current Iw2, now flows through resistor Rp, i.e., current flowing through the head is no longer Iw, but Iw1=Iw−Iw2, where Iw1>>Iw2. To a first order:
                              Iw2          Iw1                =                  Rh          Rp                                    EQ        ⁢                                  ⁢                  (          1          )                    
Note that the current loss of Iw2 is also a function of the silicon wafer process due to inherent process variation of Rp. Because Rp is an internal on-chip resistor, while resistor Rh comes from an external “head” component, the Iw1-to-Iw2 ratio will change with process. As a result, the unpredictability of Iw1 will add uncertainty and cause degradation to the write process.
A schematic of one conventional DAC implementation establishing write current Iw is shown in FIG. 3.